How can I make a mesh glow over time with keyframes?
Only properties can be keyframed

The action of "adding an emission shader" cannot be keyframed. That in not being recorded on the timeline.

When you add a node to a nodetree, that would be there for the whole scene. You can instead keyframe the factor of a mix shader ruling the shading of an object, for example between a transparent shader (so the object will be invisible) and an emission:



An alternative could be to animate the renderability of your glowing object by keyframing the little icon in the outliner. It depends on your project needs.